The Economics: Simple Payback
Turn an energy saving into a financial decision with the most-used metric in the industry — and learn where it misleads.
10 min read · Jacob Willis, Net Zero Lead · Last reviewed July 2026
An energy-saving idea only becomes a decision when someone can see what it costs and what it returns. The first tool everyone reaches for is simple payback, the most-used metric in the industry, precisely because it is so easy to grasp. Knowing both what it does well and where it misleads is what keeps you out of trouble when the numbers get large.
The formula
Simple payback (years) = Capital cost ÷ Annual saving
That is it. If a project costs £8,000 and saves £2,500 a year, the simple payback is 8,000 ÷ 2,500 = 3.2 years. After that point, the savings are pure benefit.
Try it yourself
Drag the sliders to see how cost and savings drive the payback period. Notice how sensitive the answer is to the annual saving: small changes in the estimated saving move the payback a lot, which is exactly why good measurement matters.
Simple payback calculator
Adjust the figures to see how the payback period responds.
Moderate payback — typically attractive.
Why everyone uses it
- Intuitive: anyone can understand "it pays for itself in three years."
- Fast to calculate: ideal for screening a long list of opportunities.
- Risk-aware in a rough way: a shorter payback means less time exposed to things changing.
For ranking quick wins, simple payback is hard to beat.
Where it misleads
Simple payback has real blind spots, and knowing them keeps you out of trouble:
| Limitation | Why it matters |
|---|---|
| Ignores savings after payback | A project paying back in 4 years but lasting 20 looks identical to one lasting 5 |
| Ignores the time value of money | £1 saved next year is not worth £1 today |
| Ignores energy price changes | Rising prices make savings grow over time |
| Favours short-term projects | Can starve high-value, longer-payback investments |
Because it ignores everything beyond the payback point, simple payback systematically under-values long-lived investments like insulation, building fabric or heat recovery. Use it to screen, not to make the final call on big decisions.
What comes next
When a decision is big enough to need more than a screen, you move to methods that account for the whole life of the project and the time value of money: net present value (NPV), internal rate of return (IRR) and life-cycle costing. These are the subject of the Level 2 course, Economic Analysis of Engineering Projects, which shows exactly how a project that looks fine on payback can still destroy value once time is accounted for.
A practical rule of thumb: use simple payback to screen and rank, then run NPV on the shortlist before committing real capital.
Sources and further reading
- Carbon Trust guides and tools on appraising energy efficiency investments.
- HM Treasury Green Book on project appraisal beyond simple payback.
- Energy Institute on investment analysis for energy projects.